1. Field of the Invention
The present invention relates generally to the cementing of tubular casings into well boreholes and, more particularly, to the pre-cementing treatment of boreholes to improve the integrity of subsequently applied casing cement.
2. Background Discussion
In the drilling of deep wells for the recovery of such fluids as crude oil, natural gas, and geothermal steam or brine, two or more concentric and radially-separated, tubular steel casings are usually cemented into upper, stepped-diameter regions of the borehole. These casings ordinarily extend from ground level to different depths; as an example, the outermost casing may extend to a depth of a hundred or so feet and the innermost casing may extend to a depth of many thousands of feet. In a production (as opposed to an injection) well, after the well casings have been cemented into the wellbore, a fluid-extraction pipe string is hung from the wellhead inside the innermost casing and usually extends at least to the depth thereof. In the case of a so-called "barefoot" well, a lower, uncased portion of the wellbore usually extends to a depth substantially below that of the production pipe string.
A principal objective of cementing casings to substantial depths in wellbores is to seal off earth formations penetrated by the borehole from one another, for example, to seal off higher, non-producing formations from lower formations from which fluid is produced from the well. This sealing off of underground formations from one another along the borehole is desirable for the economic reason of not losing significant amounts of the produced fluid into other formations, and may be a legal requirement to prevent contamination of the fluid--such as potable water--in one formation by fluids--such as crude oil--from another, higher pressure formation. The stepped casing construction, employing several concentrically installed and cemented casings near the earth's surface, provides high strength to protect the production pipe against damage by ground movement and counters the pressure of some penetrated formations. In addition, for geothermal wells and for some heavy oil wells in which the oil has been heated to reduce its viscosity, the added thickness provided by several concentric rings of casings and cement provides good thermal insulation for the production pipe and produced fluid.
To enable tubular casings to be cemented into place, the region of the well borehole which is to receive a casing is bored to a somewhat greater diameter than the casing diameter, thereby providing an annular cementing space between the casing and the borehole. For the same reason, the casing borehole is drilled to a somewhat greater depth than the casing length (vertical height) to provide clearance between the bottom of the casing and the bottom of the borehole. Before being cemented in place, the casing is hung in the oversized borehole with radial and bottom separation between the casing and the borehole. To prevent the formation of cement voids, the casing cement slurry is normally pumped into the borehole through the inside of the casing, the combined pumping and hydrostatic pressure forcing the cement slurry outwardly under the bottom end of the casing and back upwardly through the annular space between the outside of the casing and the borehole wall (and the inside of any next-outer casing) until the cement slurry overflows the casing at the wellhead region.
Following the injection of the cement slurry to fill the annular space around the casing, a mechanical "top plug" is installed which automatically latches into the lower end of the casing to prevent the cement slurry from flowing out of the annular space between the casing and the borehole and back up into the inside of the casing. The inside of the casing is also then typically filled with a displacement fluid to help hold the top plug in place against the hydraulic pressure of the cement outside the casing.
When the cement used to cement an outermost casing in place has sufficiently cured, the borehole for the next-inner casing is drilled downwardly through the inside of the outermost casing and its top plug to the desired depth, and the associated casing is cemented in place in the above-described manner. This well boring and casing installation and cementing process is repeated until all the casings have been cemented into the wellbore.
Detailed processes for such casing cementing are known in the art, as are compositions of the cements used. Regarding these casing cements, hydraulic cements, such as Portland cement, are commonly used. Typical cements for cementing casings are API (American Petroleum Institute) classes "G" or "H," the class "G" cement tending to be used more frequently in western regions of the United States and the class "H" cement tending to be used more frequently in the southern and Gulf regions of the United States. Both the class "G" and "H" cements are described in API Specification No. 10, such Specification being incorporated hereinto, in its entirety, by specific reference.
Difficult problems are, however, often associated with wells in which cemented casings are used. For example, well boreholes frequently penetrate porous and/or unstable underground earth formations into which the casing cement slurry can flow and be lost, thereby resulting in higher cementing costs and the possibility of undesirable cement voids. Other underground regions may be penetrated which absorb water from the casing cement slurry, thereby adversely affecting the cured strength of the cement. To overcome these and other casing cementing problems, particular types of cements have been developed, the slurries of which contain various additives--such as fiberous materials or water loss preventers--which tend to seal porous, weak, and hydroscopic formations and thereby reduce cement slurry and/or water loss into penetrated formations or earth regions.
Another, relatively common problem--and one to which the present invention is principally directed--is caused when a wellbore penetrates underground formations containing such pressurized gases as natural gas, carbon dioxide, hydrogen sulfide, methane, ammonia, or mixtures thereof. When casings are cemented through such formations, the pressurized gases from the formation can penetrate into, and bubble through, the unset cement slurry, thereby causing permanent channels (commonly called "wormholes") as the cement sets up. Any of these wormholes which extend from the borehole to the surface of the cement, or through the cement to the wall of the casing, provide a pathway for the escape of gases--and of other fluids--from penetrated formations to the wellhead region and/or to other, lower pressure formations. The escape of gases and liquids usually starts soon after the casing cementing operation is completed, but may be delayed for some time after completion of the cementing operation if the wormholes are not completely open to the surface or casing when the cement cures.
Gases and other fluids which escape into the wellhead region (through the casing cement or along casing or borehole walls) can, even at best, create inconvenient wellhead working conditions. Depending, however, upon the nature and amount of such gases and/or other fluids which leak from the wellhead region, hazardous wellhead working conditions may be created, and certain escaping liquids, such as brines, can cause excessive corrosion damage to exposed wellhead piping and equipment. Moreover, pollution standards regulating the amount of noxious gases (such as hydrogen sulfide and methane) which may be discharged into the atmosphere and the amount of polluting liquids (such as crude oil and brines) which may be discharged onto the ground, into drainage ditches, or into non-industrial sewers may be exceeded.
In situations where the leakage of fluids at the wellhead cause hazardous working conditions, wellhead pipe and equipment corrosion, and/or environmental pollution, substantial--and usually very costly--measures are necessary to correct the problem. As an illustration, the stopping of serious wellhead fluid leaks from underground formations (after the well casings have been cemented into place) is commonly attempted by boring a number of holes around the wellhead to the depth of the formation suspected of being the source of the wellhead leakage. If more than one underground formation may be the source of the leaking fluid, holes are drilled to the depth of each such formation. A cement slurry is then pressure-injected into the drilled holes to attempt to seal-off the suspected formations from the wellhead region. Such "squeeze-cementing" operation are, for example, disclosed in U.S. Pat. No. 3,242,986 to W. F. Hower. However, squeeze-cementing around a wellhead to stop the leakage of fluids is very costly--often as much as a half million dollars per well.
As an alternative, or in addition, to injecting a cement slurry through holes bored around a leaky wellhead, a downhole region of the casing installation may be perforated, by a perforating gun, in the region of a each suspected formation. A cement slurry is pressure-injected from the inside of the casing outwardly through the casing perforations into the suspected formation in an attempt to seal off the formation from the borehole.
It has, however, frequently been experienced that neither of these injection-cementing operations permanently stop fluid leakage from leaky wellheads. This may be because during the injection process or while setting up, the injected cement becomes contaminated with dirt and/or formation fluids which cause the cement to deteriorate in time. Consequently, follow-on, costly squeeze-cementing operations are sometimes performed--although such re-cementing operations are, as a rule, ineffective at stopping wellhead leaks.
It can, therefore, be appreciated that assuring the integrity of the casing cement against fluid leaks is greatly to be preferred over later attempts to stop wellhead leakage caused by casing cement channels. Toward this end, various casing cement formulations, using special additives, have been used, or proposed, for preventing pressurized formation gases from making wormholes through, or otherwise damaging, casing cement while the cement sets up. The addition of a foaming agent to a casing cement slurry prior to injecting the slurry into a wellbore is, for example, described in U.S. Pat. No. 3,926,257, to J. Marrast, et al. The disclosed foaming agent is intended to cooperate with connate gas escaping from a penetrated formation to create a foam barrier to the migration of the gas through the cement long enough for the cement to set up. Alternatively, U.S. Pat. No. 4,304,298, to D. L. Sutton, discloses the addition to, or the generation of a gas in, a casing cement slurry to prevent connate, pressurized gas from a penetrated formation from penetrating into the cement slurry.
Such special casing cements are, however, often very costly in the large amounts required for cementing casings in many wells. Furthermore, it may be very difficult to determine in advance how much preventive additives are needed for any particular cementing operation. Consequently, much greater amounts of additives than are actually needed must usually be provided in the cement slurry, thereby resulting in unnecessary costs. Still further, since the setting up of cement involves complex mechanisms, it may be very difficult to find additives which effectively prevent damage to the casing cement by formation gases during the cement set up period, but which do not adversely affect the physical properties of the cured cement during its projected lifetime.